You don't know how machine learning works. Although the ability to apply negative reinforcement would be useful (i.e. you failed to recognize that guy), purely positive reinforcement is sufficient in most simpler cases of identification, such as facial recognition. Although, that does somewhat assume that a person changes their appearance routinely enough for the machine learning to pick up on their different "looks", or to at least not overtrain on a single look. If not, it may actually lock them out with any significant change (similar to how wet fingers would typically not work with Touch ID). This would take time though, so time will tell if Apple designed this to avoid that or not.